Sign in with Apple shows a "Sign Up Not Completed" error on iOS and MacOS devices. The process works fine on other devices or when not using Safari on MacOS, but I couldn't find what this error concretely means and most people seem to have it resolved with time. Any help welcome.
For example this thread is about the same error but has no clear resolution: https://forums.developer.apple.com/thread/122458 (we already encode parameters that need encoding).
